linux-kernel - Re: [PATCH] watchdog: revert cleanup handling of false positives

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Message-ID: <YKXv/CtdWwiqoAHa@google.com>
Date:   Thu, 20 May 2021 14:13:32 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: revert cleanup handling of false positives

On (21/05/18 17:36), Petr Mladek wrote:
> On Mon 2021-05-17 23:06:12, Sergey Senozhatsky wrote:
> > This reverts commit 9bf3bc949f8aeefeacea4b1198db833b722a8e27.
> > 
> > I can reproduce the case when resumed VCPU starts to execute
> > is_softlockup() with PVCLOCK_GUEST_STOPPED set on this VCPU:
> > 
> > 	watchdog_timer_fn()
> > 	{
> > 		...
> > 
> > 		kvm_check_and_clear_guest_paused();
> > 
> > 		...
> > 
> > 		duration = is_softlockup(touch_ts, period_ts);
> > 		if (unlikely(duration)) {
> > 			....
> > 		}
> > 	}
> > 
> > Which means that guest VCPU has been suspended between
> > kvm_check_and_clear_guest_paused() and is_softlockup(),
> > and jiffies (clock) thus shifted forward.
> 
> Are jiffies really updated here?

I guess so. Why not?

VCPUs are not brought up simultaneously, it's up to the host that
schedules them. So, for instance, when we resume VCPU-3 and it
discovers this_cpu PVCLOCK_GUEST_STOPPED the VCPU-0 can already
be resumed, up and running, adding ticks to jiffies.

Am I missing the point of your question?